RESTON, Va. – If Loudoun County walks away from the Dulles Rail project, it could be years before the next phase gets back on track.

That means more years until a Metro train finally makes it to Dulles International Airport.

That sobering message was delivered Wednesday by Scott York, chairman of the Loudoun County Board of Supervisors.

“It’ll probably delay the project up to three years,” York told a room full of business leaders, developers and elected officials during a conference hosted by the Greater Reston Chamber of Commerce. “I think if Loudoun County opts out it will be a huge mistake.”

Loudoun County leaders recently said they need more time to make a decision about whether they are in or out for Phase 2 — the section planned to run from Reston to Dulles International Airport and continue into Loudoun County.

Costs to the county are the major concern. The total cost for the second phase is estimated to be around $2.8 billion. Loudoun County would be on the hook for about $260 million of that total.

If Loudoun does pull out of the project, it would essentially be like going back to square one, Virginia Transportation Secretary Sean Connaughton told reporters after the conference.

“All the financial documents, the environmental documents, the project scoping, the approvals, are all the project going out to Loudoun County,” Connaughton said.

York told the crowd if the county doesn’t invest in Dulles Rail, the county will not hit its full potential.

If all parties agree to proceed, Metropolitan Washington Airports Authority says the contract could be awarded in January 2013, and the next phase could be built and open to riders in 2018.

All parties involved are trying to keep the overall cost of the second phase down because higher costs could lead to much higher tolls on the Dulles Toll Road, which will account for 75 percent of the financing.

WTOP’s Adam Tuss contributed to this report. Follow Adam and WTOP on Twitter.